1. Field of the Invention (Technical Field)
The invention relates to oscillator circuits and more particularly to oscillator circuits which operate with high loss resonators such as resonator sensors immersed in a liquid medium.
2. Background Art
This invention is an improvement over existing technology because of its simplicity, versatility, and functionality over existing devices. The circuit of the present invention can be used with a wider variety of sensor resonators (resistance and operating frequency) than is possible with current designs while not giving up the ability to measure the loss of the resonator in a simple manner. Additionally, the circuit can easily be adapted to a wide variety of applications.
Standard oscillator designs, like the Pierce or Colpitts type, are good for frequency source oscillator designs which provide low phase noise and high over-all frequency stability. These designs do not work well for sensor applications because of their inherent phase and gain sensitivity to resonator loss, R.sub.m.
This design is technically different from existing oscillator designs designed for similar applications for the following reasons:
1. This design provides frequency and loss information in a grounded resonator configuration with a minimum of amount of complexity. No previous designs will allow frequency and loss measurements in a grounded resonator configuration with a simple design as presented, this is due to the fact that this oscillator exploits the loss of the resonator and uses the resonator impedance as in a divided form via the transistor feedback function. With the resonator impedance effectively decreased by feedback the oscillator can easily be controlled by high loss resonators. PA1 2. This oscillator configuration combines many desirable features into one design:
a. AGC controllable, for measuring R.sub.m ; PA2 b. wide bandwidth capable, high frequency resonators (&gt;100 MHz) are operable in this design; PA2 c. simple design, low parts count, no unusual or expensive components required; PA2 d. grounded resonator configuration, test fixturing is simple and practical for many sensor application; PA2 e. oscillator operates at series resonance of resonator, this allows the widest dynamic range of resistance at lowest susceptibility to fixture capacitance problems.
U.S. Pat. No. 4,661,785 to Benjaminson discloses a balanced feedback oscillator that uses effective Q multiplication which yields lower phase noise oscillator for frequency source. This circuit cannot be used for high loss sensor resonator applications because the oscillator does not independently control phase and gain. Additionally, Benjaminson has a Miller effect which severely limits the frequency range possible for high accuracy operation. Although this design can operate a high loss resonator, it cannot actively or accurately control the frequency of oscillation to a predetermined resonator condition f.sub.s or impedance phase, over a wide range of resonator loss.
U.S. Pat. No. 3,986,145 to Hongu et al., and U.S. Pat. No. 5,025,231 to Schwartzbach disclose Colpitts type oscillators that do not utilize a series resonant frequency (f.sub.s) design nor are the phase and gain functions independent for use as high-accuracy wide-dynamic range sensor oscillators.
U.S. Pat. No. 4,914,406 to Ohkubo is not a f.sub.s operating circuit and does not provide resonator loss information.
U.S. Pat. No. 5,113,153 to Soyer describes a Pierce oscillator design. This design along with Colpitts type designs are not adequate for high loss resonators because the oscillator frequency of operation is relatively sensitive to the resonator loss. These circuits are not useable in sensor-resonator applications because phase conditions and gain are dependent on each other.